In the past, in order to prevent a collision accident and a rear end collision accident, the following has been carried out. That is, an obstacle ahead of a vehicle is detected, and when the vehicle is approaching the obstacle, with the distance from the vehicle to the obstacle exceeding a threshold value, a braking force is applied to the vehicle in a forced manner, thereby reducing the speed of the vehicle. On the other hand, when the driver recognizes approaching the obstacle and is going to avoid the collision with the vehicle by carrying out avoidance steering, if a braking force is applied to the vehicle in a forced manner without taking the avoidance steering into consideration, the braking of the vehicle due to the braking force and the avoidance steering may interfere with each other, so that a sufficient avoidance effect has not often been obtained. Accordingly, there has been disclosed a technique in which a maximum braking force that can be generated on the vehicle is calculated from a lateral force generated by the avoidance steering of the driver, by making use of a so-called circle of friction, and a maximum value of the braking force is set to this calculated value, thereby making it possible to carry our effective collision avoidance (see, for example, Patent Document 1).
In addition, there has been disclosed another technique in which in order to avoid interference between the braking of the vehicle due to the braking force and the avoidance steering, the braking force to be applied is decreased under the condition that such interference is likely to occur (see, for example, Patent Document 2). In such a technique, in cases where collision avoidance by steering is possible, the braking force to be applied is set lower in comparison with the case where it is judged that the collision avoidance by steering is impossible. This attains effective collision avoidance.